With time coming into 21th century, and with the development of scientific and technology and forthcomingness of information society, especially with the widespread promotion of kinds of mobile communication devices, electronic equipment, office automation products, domestic appliances and medical devices, people have greater demand on energy. At the same time, to alleviate conflict of human being with nature and seek for sustainable development, it has become a rigorous challenge in the 21th century for human to protect nature environment and nature resources. Therefore, it has been a major subject for various countries to develop and exploit new energy and new materials.
Electric energy is very important and indispensable energy for our everyday life and work. Use of any other resources will rely upon electric energy. Also, preservation, conversion and transportation of electric energy all involve battery technique.
In present information era, people have higher and higher requirement for power supply performance, in addition to increased requirement for amount of power supply. The requirement mostly includes high power density, high specific energy, long cyclic lifetime, and large capacity. Also, higher requirement has been imposed to power supply in terms of safety, cost and environment friendship. Conventional battery such as lead acid battery, Ni—Cd battery and Ni-MH battery suffers from drawbacks such as short lifetime, low energy density and environment pollution, thus greatly limiting their use. As lithium-ion battery bears good electrochemical performance, it has become popular new high energy green battery.
Lithium-ion battery is a new type of battery developed from lithium battery. When compared with lithium battery, the most significant advantage of it lies in: material obtained by lithium ion intercalation and de-intercalation may be used to replace lithium, thus resolving problems of lithium anode passivating and dendrite penetration. In addition to maintaining high capacity and high voltage of lithium battery, charging-discharging efficiency and cycle life of it are also improved significantly. Moreover, the safety of battery is also enhanced.
Currently, ordinary material used as anode of lithium-ion battery generally includes layered lithium-intercalated compound LiMO2, spinel-type lithium intercalated compound LiM2O4, and olivine-type lithium intercalated compound LiMPO4.
LiCoO2 and LiNiO2 is common layered lithium intercalation compound. As anode material, LiCoO2 has high lithium intercalation potential and ideally, its capacity can be up to 274 mAh/g. In actual cycle period however, when hale number of lithium ions is extracted, capacity of the material will be decreased expressly, thus leading to tendency of collapsing of its layered structure. As such, the actual capacity is no more than 150 mAh/g. Furthermore, resource of Cobalt (Co) is rare, expensive, and has certain toxicity. Therefore, some active material with comprehensive electrochemical characteristics, wide availability, and low cost, must be developed to replace Co. Theoretically, LiNiO2 anode material has specific capacity of 275 mAh/g and actually is can get up to 190-210 mAh/g evidently higher than LiCoO2. Accordingly, it is regarded as one of most prospective anode materials for lithium-ion battery following LiCoO2. However, LiNiO2 has some disadvantages limiting its application range, such as rapid decrease in cycle capacity, bad thermal stability and the like.
LiMn2O4 Anode material is typical representation of spinel-type lithium intercalation compound, and its theoretical capacity is 148 mAh/g, and its actual capacity is about 120 mAh/g. Though LiMn2O4 has advantages of low cost, non-toxicity, and good safety, it has unstable lattice structure and capacity attenuation in charging-discharging cycle especially under high temperature of 55° C., thus hindering its development and application.
As a most common lithium-ion battery anode material, olivine-type lithium intercalated LiFePO4 has a series of advantages including high theoretical specific capacity (about 170 mAh/g), low cost, good environment friendship, long cycle lifetime, high thermal stability, and safety. Due to these advantages, it has become hot topic to be researched and developed in present battery industry, and has been expected to be a commercialized lithium battery anode material. As a powerful battery, LiFePO4 lithium battery will necessarily become alternative of other types of lithium batteries such as lead acid, Ni-MH and Ni—Cd batteries. Accordingly, LiFePO4 lithium battery has been considered as a mark of new era of lithium-ion battery.
At present, ordinary LiFePO4 lithium battery uses graphite as cathode due to its high specific capacity and low and steady discharging. However, as potential of carbon cathode is close to a standard potential of lithium, in case of battery overcharging, metal lithium may be crystallized on the surface of the carbon electrode and resulting in short circuit. Further, majority electrolyte becomes unsteady under this potential and, the electrolyte is subject to decomposing on the electrode surface and therefore, causes generation of mixture of inflammable gases and presents potential safety problem. In addition, insertion of Li+ into the carbon electrode will result in volume deformation in amount of 10%, causing discontinuity among particles. It further causes loosening and peeling off of the interface between the electrode and electrolyte and between the electrode and current-collector. These factors urge the researchers to make decoration and modification to the present cathode materials and continuously seek for novel lithium-ion battery cathode material with good property, simple manufacture process and low cost. Results of nail test made upon 100 Ah cylindrical LiFePO4/C lithium-ion battery show that the battery surface temperature can reach 200° C. Apparently, regard to large capacity powerful battery of 100 Ah and above, it is desired to obtain highly safe and steady cathode material.
Li4Ti5O12 is an ideal intercalation of electrode material. Intercalation and de-intercalation of Li+ has little impact on material structure and therefore, it is called “zero deformation” material. The potential of Li4Ti5O12 relative to the lithium electrode is 1.55V (relative to Li/Li+), the ideal theoretical capacity is 175 mAh/g, and experimental specific capacity reaches 150-160 mAh/g. It also has good cycle property, long and flat discharging feature, clear voltage rapid change at the end of charging and discharging period, high intercalation lithium potential without crystallization of lithium, capability of being utilized between stable voltage range of most liquid electrolytes, high Coulombic Efficiency (close to 100%), wide source availability, clean and environment friendly feature. Accordingly, it has features required by the next generation of lithium-ion battery such as much more charging repetition times, rapid charging speed and high safety. Comparatively, nail test for 100 Ah cylindrical LiFePO4/Li4Ti5O12 lithium-ion battery shows that the battery surface temperature is only 40° C. It is clear that the use of Li4Ti5O12 as cathode material improves safety of large capacity powerful lithium-ion battery and promotes commercialization of it.